A while ago I used to do business with a guy who could sell me pretty high amounts of gift cards at 30% of the value for bitcoin, after a while I got him to fess up on how he was getting them.
Turns out he was hitting up the local home depot self checkout with credit card dumps he bought online, and picking up 100 $50 iTunes cards in less than 2 hours.
I ended up referring the guy to someone with higher risk tolerance, but I hope that gives you a better understanding of the scale. That's simply not possible with conventional checkouts.